I am currently working on designing a Common Source amplifier with NMOS transistors. Unfortunately, I've had a few problems and don't seem to be making any progress. The requirements for the circuit are:

  • Driving a load of 500 Ohms
  • Maximum drain current of 100mA
  • Maximum (W/L) of 1000
  • Gain of 20dB (10V/V) at 10kHz
  • Input is a 0.1V sine wave at 10kHz with a series resistance of 25kOhms
  • Only source available is Vdd@5V (pictured) and GND

Furthermore, my given information is as follows: k = 10^-5 A/V^2, Vt (threshold voltage) = 1V, Lambda = 0.02

Now for what I've tried: enter image description here

Here, I've substituted many different values for R5 and R4, but my general approach is as follows.

  1. Obtain the Q-Point values of Vgs and Id
  2. Anaylze the circuit via small signal equivalent - doing so, I arrived at the equation Req = 1/((1/R6)+(1/R5)), where Av = (-𝓰ₘ)(Req), 𝓰ₘ being the transconductance value (which itself is equivalent to sqrt(2*k*Id_q)).
  3. I calculated the input resistance as R2||R3 to obtain Rg = R2||R3 = 50kOhms.
  4. For the output resistance, I disconnected the load and assumed the drain resistance to be arbitrarily large so that the output resistance, Ro = R5.

None of my solutions have yielded proper results and I was wondering what I'm doing wrong (any help is appreciated). Thanks.

  • 1
    \$\begingroup\$ Step 2 should be step 1, the gain Av isn't determined only by the parallel value of R5 and R6 but also by R4. Determine a formula for Av with gm, R5, R6 and R4 as inputs. Note how the ratio Req/R4 is important. Pick a reasonable value for R5 like 100 ohms and see what R4 needs to be and what gm you need. Then do step 2 (your step 1) and determine Ids and Vgs from gm. \$\endgroup\$ Aug 28, 2018 at 6:09

1 Answer 1


Consider this as a start


simulate this circuit – Schematic created using CircuitLab


Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.